The present invention relates to a method and apparatus for forming optical fiber preforms, especially those preforms that contain dopants that are not readily incorporated during the deposition process.
Optical fibers have been routinely fabricated on a commercial basis with losses less than 1 dB/km in at least part of the optical region of the spectrum, generally extending from 0.7 to 1.7 microns. The fibers comprise a core and a cladding, with the cladding having an index of refraction lower, at least in part, than that of an index of refraction associated with the core. Such low loss optical fibers are formed of glass comprising primarily silica, i.e. the glass composition comprises more than 50% silica.
Dopants which are used to make optical fibers include germania, an index raising dopant, which is the principal and most widely used dopant, as well as other minor dopants, such as phosphorus, and other index raising dopants, and fluorine and boron, index lowering dopants. Other dopants considered for use in optical fibers include Al, Zr, Nb, Ta, Ga, In, Sn, Sb, Bi, the 4f rare earths (atomic numbers 57-71), and the alkaline earths Be, Mg, Ca, Zn, Sr, Cd, and Ba. Of these, certain rare earth-doped optical fibers are of interest for a variety of applications including fiber lasers, attenuators and sensors.
Optical fibers are normally made by the oxidation of metal chlorides. Chlorides are conventionally used because they can be vaporized at relatively low temperatures and delivered to a hot zone where they are oxidized. By "hot zone" is meant that region of a glass preform forming apparatus where glass forming reactant vapors are oxidized; it can include, for example, a region within a burner flame or a heated region within a substrate tube. Vaporization techniques typically used for silicon tetrachloride and germanium tetrachloride include bubbling, direct vaporization and flash vaporization. Other chlorides that have been used commercially include boron and phosphorus chlorides which are also liquid or gaseous at room temperature.
There are however several other metal chlorides that could be used in optical waveguides that are solids at room temperature and may or may not sublime rather than boil. These properties make it nearly impossible to deliver these materials with conventional systems. Reactants for forming such other metal oxides have been formed by vaporization of the metal chloride (U.S. Pats. Nos. 3,801,294, 4,604,118 and 4,787,927) and by reaction of a halide such as chlorine with the dopant metal to form a chloride that is delivered to the hot zone through a heated injection tube (U.S. Pat. No. 4,616,901).
In the MCVD process (U.S. Pat. No. 4,217,027), silica and/or doped silica particles are formed in the 1800.degree. C. hot zone; they then flow downstream where they deposit on the tube wall. Although a relatively short time is required for sintering each layer of particles, typical sintering temperatures are 1800.degree. C. Since each layer is separately sintered in the MCVD process, all previously applied layers are subjected to temperatures on the order of 1800.degree. C. during the sintering of each subsequently deposited layer. U.S. Pat. No. 4,616,901 points out the tendency of the nonglass forming refractory oxides to crystallize if given sufficient time, at sufficiently elevated temperatures. To prevent devitrification of silica fibers containing dopants such as alumina and zirconia, that patent teaches that an effective amount of phosphorus oxide can be added to the core glass during the manufacture of the silica preforms. Optical fibers having silica cores doped with Al.sub.2 O.sub.3 and P.sub.2 O.sub.5 and having numerical apertures of 0.16 and 0.27, exhibited minimum losses of 2 dB/km and 8 dB/km, respectively, at 1.15 .mu.m. These losses were considered to be relatively low, partly because of the reduced tendency of the ternary Al.sub.2 O.sub.3 -P.sub.2 O.sub.5 -SiO.sub.2 glass system to devitrify. However, for certain purposes, the incorporation of P.sub.2 O.sub.5 in the core of an optical fiber is undesirable.
The flame hydrolysis process is similar to the MCVD process in that the glass particles are subjected to relatively high temperatures during the particle formation and deposition stage of the process. In the flame hydrolysis process (U.S. Pat. No. Re. 28,029) silica and/or doped silica particles that are formed in a high temperature flame are deposited on a temporary mandrel, and the deposited particles are subjected to elevated temperatures during the immediately subsequent traverses of the particle-generating flame along the preform. The mandrel is removed, and the resultant porous tubular preform is sintered to a clear glass tube at about 1450.degree. C. Optical fibers having silica cores doped with 18 wt. % GeO.sub.2, 1.5 wt. % Al.sub.2 O.sub.3 and 800 ppm Er were formed by the flame hydrolysis process, all reactants being delivered to the burner as chlorides. These optical fibers exhibited minimum attenuations between 5 and 10 dB/km at wavelengths between 1300 and 1550 nm.
Various techniques have been developed for incorporating dopants into porous tubular flame hydrolysis-produced preforms after the preform deposition stage; see for example, U.S. Pats. Nos. 3,859,073 and 4,263,031, both to P. C. Schultz. Such dopants are therefore spared the high temperatures encountered during the initial preform formation process.
In accordance with the teachings of U.S. Pat. No. 3,859,073, a porous preform formed by the flame hydrolysis process is cooled and then immersed in a solution containing a dopant. The porous preform is dried and heat treated to consolidate or sinter it into a non-porous glass body containing the dopant. Solution doping techniques are time consuming in that they require the steps of immersion and drying in addition to the conventionally performed steps of deposition and consolidation. Moreover, porous preforms have often been rendered useless due to either disintegration during immersion in the solvent or cracking of the outer layers of the preforms during drying. Larger preforms, which are preferred for use in commercial operations, exhibit a greater tendency to fracture during immersion.
U.S. Pat. No. 4,263,031 teaches a method of flowing a dopant chloride into the aperture of a porous tubular preform while the preform is in the consolidation furnace. The dopant chlorides disclosed in that patent are the aforementioned conventional chlorides such as chlorides of germanium, phosphorus, titanium, which can be vaporized at relatively low temperatures and delivered from a source such as a bubbler. Chlorides of metals such as aluminum, zirconium and the like, which can be supplied through a heated delivery system to an MCVD substrate tube in accordance with the teachings of U.S. Pat. No. 4,616,901, cannot similarly be delivered to a porous preform disposed in a consolidation furnace. The system for delivering gas to the aperture of a preform in a consolidation furnace would need to be heated before it is inserted into the consolidation furnace to prevent condensation of the dopant chloride Such consolidation furnace delivery tubes are not easily provided with heating means. Furthermore, part of the delivery tube is subjected to consolidation temperatures (around 1450.degree. C.) which would adversely affect conventional heating tapes.